1. Field of the Invention
The present invention relates to a device for detecting a travel distance, an image forming apparatus, and a method for detecting a travel distance.
2. Description of the Related Art
Image forming apparatuses, e.g., printers, copiers, and Multi-functional Peripherals (MFPs): multifunction devices or combination devices, form an image onto a surface of a paper sheet while the image forming apparatuses convey the paper sheet. For example, an electrophotographic image forming apparatus forms a toner image on a surface of an image carrier to transfer the toner image onto a paper sheet while the electrophotographic image forming apparatus moves the image carrier and the paper sheet at the same linear velocity as each other. In order to control a conveyance speed of the paper sheet or of the image carrier, an image forming apparatus of this type is provided with a device for detecting a travel distance per a predetermined amount of time corresponding to the conveyance speed.
As a method for detecting a travel distance, a method is known in which a moving object is irradiated with light to make areas of light and darkness depending on irregularities of the surface of the moving object, an image of the moving object irradiated with light is captured at predetermined time intervals, and an amount of difference in position of a common pattern of the two images is detected. The method enables detection of a travel distance even if the moving object has a solid color surface, in other words, even if the surface color of the moving object is monochrome.
Conventional technologies for detecting a travel distance based on two images obtained by image capturing are described, for example, in Japanese Unexamined Patent Application Publication Nos. 2002-202705, 2013-257187, and 2014-119432.
The first publication discloses performing filtering for reducing gray levels of an image prior to matching to compare between two images, and filtering for preventing a value from suddenly changing relative to a conveyance speed detected.
The second publication discloses calculating a cross-correlation function between two image patterns (speckle patterns). For the calculation, discrete Fourier transformation is performed on the image patterns and processing of removing background is executed in a frequency space.
The third publication discloses an inspection device for determining appropriateness of conveyance speed of a sheet. The inspection device calculates a conveyance speed of the sheet on the basis of an image correlation of two speckle images picked up with a given time interval; and performs filter processing for removing a variation component of a given band width on the conveyance speed calculated.
The second publication describe the technology of detecting a travel distance by calculating a cross-correlation through discrete Fourier transformation of an image. The technology, however, involves a problem that an error occurs in the result of detection. Such an error occurs when the technology described in the second publication is used to perform the background removal processing.
The first publication describes the technology of detecting a travel distance by matching between two images (comparing between two images in a real space). The technology, however, sometimes fails to detect a travel distance when the images have too many characteristic points. In other words, the technology has a difficulty in detecting, at a high accuracy, a travel distance of a moving object of which a surface has subtle irregularities.
The third publication describes the technology of performing the filter processing after calculation of a conveyance speed at predetermined time intervals and accumulation of the conveyance speed. In order to determine appropriateness of a conveyance speed, it takes a long time to accumulate the conveyance speed as compared with the time interval for calculation of each conveyance speed. This makes it difficult to reflect the result of determination in speed control at real time.